There are many instances in which the need arises to turn an instrumentality about a selected axis and through a predetermined arc with a high degree of precision. Such instances include, for example, a telescope drive system by which rotation of the earth is effectively cancelled for astronomical observations; a device for precisely setting angular positions of optical elements in a laser system; and other angular drives which require precise indexing and/or tracking.
A common prior art drive for the purposes outlined above is the precision worm/worm gear drive. However, this type of drive is subject to a number of inherent errors. For example, the worm/worm gear drive is subject to a periodic error caused by deviations in the helical path of the worm thread. These deviations cause a sinusoidal error to occur with every revolution of the worm. This sinusoidal error is especially troublesome in astronomical observations because it causes star images formed by long-exposure photographs to become blurred and unsuitable for scientific study.
Another source of error found in precision worm gear drives, as well as in precision spur gear drives, is tooth-to-tooth position discrepancies of the gear. Also, back lash errors sometimes occur which are caused by clearance between the drive worm threads and gear teeth, or spur gear teeth, and the teeth of the driven gear.